Unlike treating elementary particles as points (zero-dimensional objects), the starting point of string theory is to consider the propagation of one-dimensional objects (which can be roughly imagined as line segments or circles) in time and space. On this basis, the equations of motion, quantization and theoretical symmetry are studied.
In the development of string theory, supersymmetry (the research object of superstring theory is supersymmetric strings), membrane and duality (different versions of the theory) are gradually introduced, which also gives birth to many mathematical problems and promotes each other with mathematical theory.
Phenomenology of string theory tries to deduce the corresponding low-energy quantum field theory (standard model) from high-energy string theory. This is a bit like the process of quantum mechanics "deducing" classical mechanics.